Adaptors for fitting pressure measuring probes to internal combustion engines are produced and distributed as ignition or glow-plug adaptors, the geometry of the glow-plug adaptor typically corresponding to a glow-plug. To adapt the glow-plug adaptor to the required geometry, glow-plug adaptors known from the prior art include adaptor tips, as is known in the case of glow-plug adaptors.
DE 102011088474 describes a glow-plug adaptor which has a metallic or ceramic adaptor tip, which is mounted on the combustion chamber upstream of a piezoelectric pressure sensor. The pressure acting on the glow pin is transferred to the pressure sensor and thereby measured.
A similar structure is described in U.S. Pat. No. 8,205,490, which is hereby incorporated herein by this reference for all purposes. Forces or pressures acting in the longitudinal direction on an adaptor tip of the glow-plug adaptor are transferred to a diaphragm of a piezoelectric sensor and measured. The adaptor tip is accordingly movably mounted and acts in each case as a pressure absorbing element, so that forces are indirectly transferred via the adaptor tip onto the diaphragm of the piezo-electric sensor. The measuring accuracy achievable by these arrangements is less than optimal, due to the indirect force transmission.
A different design is described in the glow-plug adaptor in accordance with DE 20301021. This glow-plug adaptor can also be fixed instead of a glow-plug in a glow-plug socket in a combustion chamber wall. The glow-plug adaptor has an adaptor head, which is substantially cylindrical in design, has a sealing cone and can be fixed in the glow-plug socket forming a sealing seat. The pressure sensor is embodied as a non-cooled high-temperature pressure sensor, comprising a movable diaphragm which is directly acted on by the gas pressure of the combustion chamber. The diaphragm is mounted as close as possible to the combustion chamber, which increases the measuring accuracy.
The adaptor head of the glow-plug adaptor has an adaptor tip attached thereto, to simulate as accurately as possible the shape of the tip of a glow-plug. The adaptor tip is modelled on the contour of the tip of the glow-plug and is permanently fixed to the adaptor head at the combustion chamber-facing end of the glow-plug adaptor by a materially bonded connection. An adaptor tip is understood to mean a rigid device which is fixed to the adaptor head at the combustion chamber-facing end and is connected upstream of the pressure sensor on the combustion chamber side. In the installed condition the adaptor tip protrudes into the combustion chamber, analogously to the arrangement of a glow-plug. In the adaptor tip a plurality of gas exchange channels are provided, which lead into a cavity in front of the diaphragm of the pressure sensor. Since only a small gap remains in the narrow glow-plug openings between the adaptor tip and the combustion chamber wall, the heat inflow to the combustion chamber is restricted.
Depending on how far the adaptor tip protrudes into the combustion chamber, high temperatures and large temperature drops occur at the pressure sensor of the adaptor, which negatively affect both the measurement accuracy and the service life of the pressure sensor. The thermal convection around the body of the adaptor tip and in the large gap between the adaptor tip and the inner wall of the glow-plug socket results in a strong heating of the adaptor tip. To prevent a strong heating, attempts have been made to design the gap to be as small as possible.